>> reporter: scott, law enforcement tells us that dzhokhar tsarnaev has indicate he and his brother acted alone use he attack. because he was shot in the neck, the interrogation has been challenging but our sources say he has been cooperating with written answers. today prosecutors needed to hargince a magistrate there was though evidence to charge tsarnaev so they filed a sworn affidavit revealing new details. 2:38 lastdaafrnoon, 11 minutes before t first kiplosion, the tsarnaev brothers were spotted walking each with a heapsack. as they turn tonight boylston street where the marathon was being held. three minutes later they stopped and stood together for one minute. then they split up. tamerlan tsarnaev headed towards the finish line. his younger brother dzohkhar blended in with a crowd of spectators near the forum restaurant. at 2:45 dzohkhar was captured by cameras placing his backpack on the ground. over the next four minutes he spent time looking at his cell phone, app picture and holding the phone near his waist. then he raised it to his ear. 30 seconds later at 2:49 p.m. the first